An unpaid custodian, who ordinarily must swear that he was not negligent in guarding an object entrusted in his care, does not have to swear regarding these things. A paid custodian, who is ordinarily liable for theft, does not have to swear and does not pay for those things.
An oath requires a definite measure. For example, if one says, "I delivered a house full of produce to you for safekeeping," or "a purse full of money," and the guardian says, "I do not know how much you gave, but I am giving you back everything," - he does not have to swear. However, if he says that the produce reached the ceiling beam, and the guardian says it only reached the window, the guardian has to swear due to his partial admission.
Art: Woman Seated At Window by John George Brown